Pellagra, a disease of niacin-deficiency, may occur in association with the carcinoid syndrome. A 63-year-old black woman had both diseases. In the carcinoid syndrome, functioning tumor cells indirectly depress endogenous niacin production by diverting tryptophan metabolism towards serotonin and away from niacin. Anorexia and diarrhea, frequently present in the carcinoid syndrome, reduce the availability of exogenous niacin by decreasing the amount ingested and absorbed. The decreased availability of endogenous and exogenous niacin eventually results in the depressed tissue niacin levels responsible for the development of pellagra. In our patient, oral and parenteral administration of niacinamide (nicotinamide; the amide form of niacin) led to rapid clearing of the pellagrous dermatitis.